Field of the Invention
The invention relates generally to the operation of linear motors, and more particularly to means for determining whether a linear motor has reached an end stop at the end of a power or return stroke.
Related Art
In the production of oil from wells, it is often necessary to use an artificial lift system to maintain the flow of oil. The artificial lift system commonly includes an electric submersible pump (ESP) that is positioned downhole in a producing region of the well. The ESP has a motor that receives electrical signals from equipment at the surface of the well. The received signals run the motor, which in turn drives a pump to lift the oil out of the well.
ESP motors commonly use rotary designs in which a rotor is coaxially positioned within a stator and rotates within the stator. The shaft of the rotor is coupled to a pump, and drives a shaft of the pump to turn impellers within the body of the pump. The impellers force the oil through the pump and out of the well. While rotary motors are typically used to enhance oil recovery, it is also possible to use linear motors for the same purpose. Instead of a rotor, the linear motor has a mover that moves in a linear, reciprocating motion. The mover drives a plunger-type pump to force oil out of the well, thus enhancing production.
In order to properly control a linear motor, it is desirable to know the position of the mover within the stator. Linear motors may use a variety of sensors (e.g., Hall-effect sensors) to determine the position of the mover. The signals from these sensors (which may be referred to herein as the Hall signals) are provided, as a feedback signal, to the control system. This control system then produces a drive signal to move the motor in the power or the return stroke. The speed of the motor is controlled by increasing or decreasing the voltage level of the drive signal.
It would be desirable to provide means to determine whether the mover of a linear motor has stalled or stopped (e.g., when the motor's mover has reached an end stop), so that the mover may be moved in a controlled fashion in either direction.